[FIX] mrp: Unrelated product_template and product_product on the same BOM

Before this fix, we can create a Bom with both product_id and
product_tmpl_id filled where the product_id is not a variant of the
product_tmpl_id. This is unlogical and will be even trickier when the
user is not able to see the variant field on the BOM.

This fix puts the product_id back to False when the product_tmpl_id
is changed.

addons/mrp/models/mrp_bom.py

@@ -81,6 +81,8 @@ class MrpBom(models.Model):
     def onchange_product_tmpl_id(self):
         if self.product_tmpl_id:
             self.product_uom_id = self.product_tmpl_id.uom_id.id
+            if self.product_id.product_tmpl_id != self.product_tmpl_id:
+                self.product_id = False
 
     @api.onchange('routing_id')
     def onchange_routing_id(self):